Field of the Invention
The present invention relates to a heat treatment apparatus for treating a strip-shaped substrate such as a film by heating the substrate.
Description of the Related Art
Conventionally, there is a case where a heat treatment of a thin substrate formed of an elongated film or sheet is performed. For example, a degassing treatment performed before deposition by sputtering or plasma-enhanced chemical vapor deposition (plasma CVD) is known as the foregoing heat treatment. In the degassing treatment, the substrate is heated to remove oligomer or moisture, which remains in the substrate, from the substrate as gas. The substrate is heated, for example, by heat radiation from a heater to the substrate or by a direct contact between a heating roller and the substrate.
Conventionally, apparatuses described in WO2010/024018A and JP2010-053382A (which will be hereinafter referred to as Patent documents 1 and 2, respectively) are known as apparatuses each performing a degassing treatment by means of a heater. The apparatus disclosed in each of Patent documents 1 and 2 is provided with plural rollers each rotatable, plural plate heaters, and a chamber housing the rollers and the plate heaters. The plural rollers are arranged within the chamber so that their rotational central axes are parallel to one another. A film as a substrate is hung over the rollers. Each of the rollers has an outer circumferential surface, which makes contact with the film, and guides the film to be fed along a predetermined pathway while rotating in synchronization with feeding of the film. Each of the plate heaters has a heating surface. The plate heater is arranged so that the heating surface faces a surface of the film which is fed as described above, and the plate heater heats the substrate by heat radiation from the heating surface to the surface of the substrate.
The inventors of the present invention found a particular problem in a heat treatment apparatus based on the combination of plural rollers and a heater as described above that a specific one of the plural rollers is heated by heat radiation from the heater basically for heating a substrate and therefore a temperature of the roller remarkably increases. Thus, a normal heat treatment of the substrate which makes in contact with the rollers may be inhibited. Details of the problem are as follows.
In general, a roller width, i.e., a dimension of each of the rollers in an axial direction is designed to be greater than a width of the substrate (a dimension in a direction perpendicular to a feeding direction of the substrate) in order to surely prevent the substrate to be fed from protruding from the roller in the width direction. Accordingly, both end portions of each of the rollers in the axial direction are exposed while protruding outward from both ends of the substrate in the width direction thereof. Further, in a case where plural types of substrates having different widths are treated by a common heat treatment apparatus, the roller width is designed on the basis of one of the substrates, which has the largest width, so as to be greater than the largest width. Therefore, in a case where one of the substrates, which having a width smaller than the largest width is treated by the heat treatment apparatus, a portion of each of the rollers, which protrudes outward from the substrate in the width direction further increases.
Meanwhile, a width of the heating surface of the heater is generally designed to be greater than the width of the substrate for the purpose of inhibiting an uneven heating of the substrate in the width direction. Specifically, heat at both end portions of the substrate in the width direction is more easily radiated and released around the substrate than an inner portion of the substrate relative to the end portions. That is, the end portions of the substrate are easily cooled; therefore, a temperature of the end portions lowers more easily than temperatures of other portions of the substrate. In order to inhibit such a temperature difference, the width of the heating surface is generally designed to be greater than the largest width of the substrate so that radiation heat is sufficiently applied to the both end portions of the substrate in the width direction. As a result, both end portions of the heating surface in the width direction thereof protrude outward from the substrate in the width direction thereof.
Thus, the outer circumferential surface of the both end portions of each of the rollers in the axial direction and both outside portions of the heating surface are exposed while protruding further outward than the both ends of the substrate in the width direction. Accordingly, heat radiation from the both outside portions of the heating surface to the outer circumferential surface of the both end portions of a specific one of the rollers in the axial direction may directly heat the outer circumferential surface depending on the arrangement of each of the rollers. Such heating not only causes a temperature rising of the both end portions of the specific roller in the axial direction but also causes a temperature rising of a contact area of the outer circumferential surface of the roller, which make contact with the substrate, by heat transfer on the specific roller. Furthermore, a temperature of the heating surface of the heater is generally set higher than a target heating temperature of the substrate. Accordingly, the heat radiation may cause the temperature of the outer circumferential surface of the roller to increase higher than the temperature of the substrate. Thus, when an actual temperature of the substrate increases above the target heating temperature to reach a heat resistant limit of the substrate, the substrate may be thermally damaged. Such unfavorable circumstance may be worse as portions of each of the roller and portions of the heating surface, protruding outward from the substrate in the width direction while not covered by the substrate, are larger, i.e., as differences between the width of each of the rollers and the width of the substrate and between the width of the heating surface and the width of the substrate are larger.
The temperature rising of the roller due to the heat radiation from the heating surface can be inhibited by lowering the temperature of the heating surface. However, lowering of the temperature of the heating surface results in a slowdown in the heat treatment speed and thus leads to an extension of the heat treatment time to decrease treatment efficiency, therefore being inadvisable.